386 PROFESSOR GEIKIE ON THE 
surface during which the thinner connecting sheets of sandy detritus dis- 
appeared. 
The four groups of strata, above described, may be regarded as forming 
together a red sandy and conglomeratic base, of very variable thickness, on 
which lies the great flagstone series of Caithness now to be discussed. This 
latter remarkable and most characteristic development of the Old Red Sand- 
stone in the north of Scotland, was first examined and delineated with much 
skill in the Memoir of S—EpGwick and Murcuison. These authors comprised in 
their narrative the description of the two coast sections on the northern and 
eastern margins of Caithness. They regarded the traverse from Strathy to 
Duncansbay Head as an ascending section, and that from Duncansbay Head to 
the Ord as a descending one, though they admitted that each presented 
peculiarities not found in the other. From what has already been said regard- 
ing the geological structure of the region, it will be apparent that the two 
coast sections cannot properly be put in comparison. For in the first place, the 
supposed basement conglomerates at the west end of the northern section 
are but of trifling extent, and, instead of representing the thick masses 
already described as occurring towards the southern end of the eastern 
section, belong really to a higher horizon than is anywhere reached among 
the flagstones on the east side; in the second place, owing to anticlinal 
and synclinal foldings, as well as to faults, the same beds are repeated 
again and again, so that it is in some cases only from a protraction of the 
measured angles of dip that we can be sure, after several miles of coast section, 
whether we have reached a higher or a lower part of the series. In no one 
natural section do we meet with a continuous succession of the whole flagstone 
series. Fortunately, however, owing to the many arches and troughs into 
which the strata have been folded, most parts of the series are exposed in more 
than one coast section. Their varying lithological characters and thicknesses 
can thus be compared, and probably a more generally accurate table of the 
whole mass can be compiled than if the data were obtainable only from a single 
exposure. 
In endeavouring to construct such a table, much difficulty is experienced 
owing to the remarkable sameness of lithological characters throughout the 
flagstones, the want of strongly defined bands or zones which could be recog- 
nised at each successive reappearance at the surface, and the uniformity of the 
paleontological contents of this whole series of strata. No doubt when the 
county of Caithness comes to be geologically surveyed in detail, many traceable 
bands may be detected, and will then be used in unravelling the minute strue- 
ture of the county, and in forming a more accurate and detailed tabular 
arrangement of the flagstones than can at present be attempted. As a pro- 
visional grouping, it may be convenient to arrange the principal mass of the 

